It is known that the vertical tail of as aircraft is able to generate, as a result of the surface thereof, a side lift and an aerodynamic drag, and that it should impart to said aircraft the stability, the maneuverability and the balancing for every flight condition.
The surface of the vertical tail is sized so as to be able to compensate for the yaw effect created as a result of the breakdown of one of the engines of the aircraft, allowing to impart to the aircraft a sufficiently low minimum control speed should an engine breakdown, for meeting the take-off and landing performance of the aircraft. The dimensions of such a vertical tail are calculated at a low sideslip rate of the aircraft, for a control surface having, with respect to the stabilizer of the vertical tail, a maximum angular position.
Furthermore, it is known:                from U.S. Pat. No. 2,356,139, a method for enhancing the aerodynamic efficiency of a vertical tail of an aircraft, comprising a vertical stabilizer and a control surface able to rotate with respect to such vertical stabilizer. To this end, said stabilizer is enlarged with an additional back stabilizer, the length of said additional stabilizer being higher than the height at which it meets the existing stabilizer. The aim of this additional stabilizer is to enhance a coefficient indicating the yaw torque produced by the whole vertical tail as a function of the angle of yaw of the aircraft;        from EP-0,887,256, a vertical tail of an aircraft comprising, more particularly, a vertical stabilizer, a control surface comprising two sections, and a spoiler; and        from document WO-00/07875, a vertical tail of an aircraft, being provided with a plate at the upper end thereof.        
The aim of the invention is to increase the side lift of the vertical tail or to maintain the same side lift while decreasing the surface of the vertical tail. To this end, it provides a method allowing to enhance the local aerodynamic efficiency of the vertical tail throughout the height of the vertical tail, that is to bring the local value of the side lift coefficient closer to of the maximum acceptable value of the side lift coefficient beyond which there is a detachment of the air layer on the surface of the vertical tail.